There have been some grisly discoveries in London this week. During construction of a major East-West railway tunnel, workers on the city’s Crossrail project have just unearthed a 14th century plague pit, filled with at least 13 corpses. Now in London’s core, the mass grave would have been just north of its city wall in the Middle Ages, conveniently placed to offload the mounds of festering dead with which the Black Death filled London during the mid 1300s. It’s not the first macabre discovery that Crossrail workers have made. In 2011 they uncovered the resting place for 300 hapless "lunatics," inmates of the former Bedlam Hospital, whose (possibly staged) ravings made them one of 18th century London’s most popular tourist attractions.

For London-watchers, this comes as no surprise. It’s almost impossible to dig down here without pushing your spade into some skeleton, underground river or bit of Roman wall. Last month, possibly "opulent" Roman remains were discovered beneath London Bridge Station, near where a bathhouse of the same era had been unearthed two years prior. Some other finds make these remnants look positively modern. In front of the British secret service’s headquarters, it’s still possible to see remains of a prehistoric ritual jetty dating back to 1500 BC, which poke out from the Thames at low tide.

This underground legacy is fascinating for history buffs, but it can give construction workers the creeps – in 2007, workers on a London bridge site filled with old tombs complained that ghosts were blowing their light bulbs and stealing their tools. It also makes subterranean construction extremely intricate in a city that is already densely burrowed with sewers and subway tunnels. One popular urban myth, for example, is that the Tube line between Hyde Park and Knightsbridge station in West London veers sharply to avoid a too-dense knot of plague bodies.

It’s ironic, then, that the city is currently undergoing a controversial subterranean building boom. Across West London in particular, wealthy homeowners are finding the only way they can legally expand period houses protected with preservation orders is by building downwards, honeycombing rich areas with glitzy Bond villain-style catacombs. This keeps appearances at surface level as they always were, but as the Crossrail discovery reminds us, in a city built on bones, when you dig downwards what you find isn’t always pretty.

Top image: Crossrail workers uncovered at least 13 skeletons from a 14th century plague pit in London this week. Image courtesy Crossrail.